Screen Changers are known, in which a screen is placed in the conduit conveying the flow of melt to the die. In a conventional screen changer, two separate screens are provided, together with a changeover system, which may be manually or otherwise operated.
When the extruder is shut down temporarily, for example when cleaning the die nozzle or to change the colour of the material or to a different melt material, the opportunity can be taken to snap the screens over, whereby the clean screen is interposed in the conduit, in place of the dirty screen. Now, the dirty screen can be taken out, cleaned, and replaced, ready for re-use.
In some cases, it is desired to change the screen at a time when the extruder is not otherwise shut down. Designs have been made in which it is possible to change the screens while melt continues to flow. One major problem with change the screen while the melt flow continues is that the change affects the flow. Even if the change is done very quickly, the pulse caused by the change can affect the resulting extruded product.
Besides, the momentary disruption is not the only problem. Even if the short pulse disruption can be minimized, the pressure/flow characteristics of the new screen are never quite the same as those of the old (dirty) screen. As a result, the final product emerging from the die nozzle might not, after the change-over, have the same film thickness, through-flow rate etc.
Also, on the practical side, the type of screen system that permits change-over of the screens has been notoriously difficult to seal, and the problem of leakage of melt from the screen changer housing can be tiresome.
It is recognized that when the die is producing short batch runs, the screen can be changed between batches, and the screen can remain clean for the length of the batch. But for long or continuous production, the need to change the screen during operation arises; the quick change screen system is not very satisfactory, but is in widespread use for long runs because of the lack of an alternative.
U.S. Pat. No. 4,332,541 (Anders, 1982) shows an example of a prior art screen pack, in which a reverse-pressure back-flushing phase of operation is automatically incorporated into the extrusion process, and this is used to clean out debris that may have accumulated in the screen. The idea is that, as a result of the periodic back-flushing, the screen can be kept cleaner, longer.